1. Field of the Invention
The present invention relates to directing artillery fire.
2. Description of Related Art
During the first World War, most Western armies fully adopted a concept known as "indirect" fire. This technique involved deploying artillery in batteries positioned behind friendly lines under cover of hills, woods, and any other obstruction that hid the battery. A survey network connected the artillery battery in a network with forward observers and fire direction centers. Survey traverses were run to each battery in a "common grid" so that all guns in range could fire at a common target by measuring an azimuthal angle from north and adjusting the elevation of the gun barrel for the altitude and cant of the gun at its particular position.
The process of pointing a piece of artillery involved a crew setting out aiming stakes which the gunner used as a point of reference. The aiming stakes were viewed through a panoramic sight and, with the aid of an aiming instrument such as a mil scale, the angle to the target was measured off of the line determined by the aiming stakes. The entire artillery battery was aligned by setting an aiming circle over a known point which had been surveyed by the survey parties. Angular measurements were taken to each piece of artillery so that each gun could be aligned with each other with reference to a common direction line such as north. The positioning of the guns was then correlated to the coordinates of the target as derived from a map or observers.
A new historical phase began with the introduction of rapid and accurate counterbattery weaponry. Counterbattery weapons destroy artillery by identifying the artillery shells in flight with radar, determining their point of origin and returning fire or by other detection means locate the source of fire and begin counterbattery engagements. Semi-permanent "firebases" thus are not practical against modern counter battery measures. Rather, the presence of counterbattery weapons on a battlefield forces the artillery battery to "shoot-and-scoot"; in other words, the artillery battery must fire many rounds in a short period of time and then move before the counterbattery weapons of the enemy return fire. Furthermore, the artillery battery must quickly resume firing at a new location to deliver enough ordnance to be effective. For example, in any combat with the forces of the Warsaw Pact, NATO anticipates having to stop large numbers of highly concentrated, rapidly attacking armored vehicles protected by counterbattery weapons. NATO forces must be able to move quickly and shoot fast.
Despite the introduction of counterbattery weaponry, the techniques used to direct artillery fire remain essentially unchanged from the First World War. As a result, current methods of redeploying field artillery lag behind the tactical requirements imposed by counterbattery weapons. The artillery forces of NATO are at risk of either being ineffective or totally destroyed early in any combat with forces of the Warsaw Pact.
Various technological solutions have been proposed for making the artillery more nimble and maneuverable. One approach consists of an autonomous gun positioning system (AGPS) which supplies each individual gun with its position as well as a common reference direction. A present example of this type of positioning system is based on a ring laser gyro. A laser gyro senses changes in three dimensional position and continuously recomputes the direction of north, gun azimuth, and some AGPS even determine the cant of the gun. However, artillerists generally prefer positioning the laser gyro on the trunnions of the howitzer where the gunner normally determines the azimuth and elevation angles. The trunnions also absorb the recoil shock of a howitzer when it is fired. It is difficult to make a laser ring gyro that can withstand the repeated shock of a howitzer firing. In any event, laser ring gyros are basically quite expensive and delicate.
Other AGPS systems use an inertial gyro to sense spacial displacement. The inertial gyro is initially oriented with separate, independent means such as a north finding instrument. Other AGPS are "coupled" to an axle of the artillery piece so as to measure how far the gun moves. The accuracy of an inertial gyro system is limited, however, because of inherent inaccuracies in the inertial gyro as well as by errors introduced by slippage of the wheel caused by, for example, traveling over snow, mud, or sand. The limited accuracy of this type of system forces the battery to periodically disengage from combat and realign its inertial gyros. Disengaging from battle is clearly undesirable.
Another gun positioning system is the Position Location and Reporting System (PLRS) which uses a network of terrestrially based radio transmitters having a known position to locate additional stations at unknown locations. This type of system is similar to the LORAN or SHORAN systems currently used to determine the location of ships and planes. A PLRS system inevitably suffers from any number of vagaries associated with terrestrial emissions of electromagnetic radiation that compromise accuracy. Further, the electromagnetic emissions from these systems make the transmitters easy to locate and destroy.
Yet another class of positioning system is the Position and Azimuth Determining System (PADS) adopted by the armies of several nations including those of the United States and United Kingdom. PADS are similar to inertial coupled systems. Howitzers are mounted on tracks so as to bash through almost anything. PADS, however, can't cope with the shocks experienced by howitzers driving over rough terrain. Hence, PADS are normally mounted on separate survey vehicles where they receive fewer shocks since a driver can choose the best route for surveying rather than the best route for driving a howitzer to its assigned position. Like inertial systems, however, PADS also require periodic realignment and have limited accuracy. The first steps in aligning PADS are unevolved from common survey methods. And even with PADS the final steps involved in triangulating the position of each artillery unit in the battery are essentially the same as first used in the First World War: a crew member must leave the relative safety of the howitzer and enter a potentially contaminated environment to set out the aiming stakes. Moreover, each PAD may cost as much as quarter million dollars each which make PADS uneconomical to integrate into a combat vehicle.
A revolutionary new system for surveying and navigating uses the satellite system commonly known as the GeoPositioning System or GPS NAVSTAR. Eventually a constellation of at least eighteen satellites should transmit ephemeral data to enable GPS receivers on ships, planes, land vehicles or infantrymen to quickly and accurately determine to within meters their exact terrestrial position in three dimensions. The physical size and cost of GPS receivers is decreasing rapidly with the introduction of advanced 5 microchips and microprocessors.
The GPS system enables individual howitzers to ascertain their terrestrial position with simple, low cost equipment that will become substantially less complex and less expensive. In contrast, making inertial systems and PADS more accurate will also make them more complex and expensive. It is not surprising, therefore, that several concepts have been advanced for using GPS to direct artillery fire. Many proposals, however, involve only substituting a GPS station for a traditional triangulation station. The current solutions for finding north and surveying individual howitzer batteries include steps that are essentially unchanged from methods introduced in the First World War.
Some proposals have been made to take two sequential position fixes with one GPS ground receiver and then determine the azimuthal angle between the two measurements as is currently done with the PADS system. For tactical reasons, however, a howitzer is not always able to stop and measure its location. Moving the howitzer to a second position just to get a reading, or stopping to take a reading while approaching the firing position, slows the process of quickly occupying a position and opening fire. Further, a two step firing procedure may be tactically untenable.
Another proposal is to deploy two GPS antenna units along the barrel of the howitzer. The barrel of the howitzer provides the base line for determining north. This base line, however, is inconveniently short with present technology to determine direction to a sufficient accuracy. A "one gun" solution does not make use of tactical practice of dispersing artillery in units of at least two guns and requires an separate, independent backup system for the case where a GPS receiver, or the entire GPS system, fails.
The GPS satellites provide data in one of two coded forms: a very accurate P Code which is classified and limited to the US and selected Allies, and the CA code available for all civilian applications. If the CA code is used to accomplish artillery fire control, the ground stations must accommodate "differential techniques" using known locations and special mathematics known to those skilled in the art.
No known system for aiming artillery that uses GPS accommodates all these factors with tactical practicality.